1. Field of the Invention
The present invention relates to a phase shifter for use in irradiating a substrate to be treated with a luminous flux having a predetermined intensity distribution by use of interference of laser light having different phases. This phase shifter is used, for example, during laser annealing performed in order to grow crystal grains in a specific area of the surface of a substrate to be treated, when a thin film transistor is formed on the surface of the substrate to be treated.
2. Description of the Related Art
In a display device such as an active matrix type liquid crystal device or an organic electroluminescent display device, a large number of thin film transistors (TFTs) are formed on an insulating substrate made of glass, plastic or the like in order to individually drive pixels. As to an amorphous silicon (a-Si) film for use in source, drain, and channel areas of the TFT, since a forming temperature is low, the film can be comparatively easily formed by a vapor phase growth process, and mass productivity is also satisfactory, the film has been broadly used as a semiconductor thin film for forming the TFT.
However, the amorphous silicon film has a disadvantage that the film is inferior to a polycrystal silicon (poly-Si) film in physical properties such as conductivity (mobility of a-Si is lower than that of poly-Si by two or more digits). Therefore, it is necessary to establish a method of forming the source, drain, and channel areas of the TFT in the polycrystal silicon film in order to increase an operation speed of the TFT thereafter.
In the present situations, as a method of forming the polycrystal silicon film, for example, there is used an annealing process (hereinafter referred to as the excimer laser annealing [ELA] process) using excimer laser. The ELA process is applicable to various processes other than the forming of polycrystal silicon, when an average intensity (fluence) of the laser light is changed. For example, when the intensity of the laser light is set to a region having an only heating function, the process is usable in an impurity activation process required for forming the TFT. When the intensity of the laser light is excessively increased, a rapid temperature rise is caused, and therefore the process is usable for removing the film from the TFT. It is to be noted that the use of these phenomena is not limited to the TFT, and the phenomena are broadly applicable to a semiconductor manufacturing process.
This ELA process can be carried out in a temperature region (i.e., from room temperature to about 500° C.) in which a versatile glass substrate is usable (Matsumura, Surface Science Vol. 21, No. 5, pp. 278 to 287, 2000). In the ELA process, for example, after depositing an amorphous silicon film in a predetermined thickness (e.g., about 50 nm) on a substrate, the amorphous silicon film is irradiated with krypton fluoride (KrF) excimer laser light (wavelength 248 nm), xenon chloride (XeCl) excimer laser light (wavelength 308 nm) or the like. The amorphous silicon film is locally molten, recrystallized, and changed into the polycrystal silicon film having an average particle diameter of about 0.1 to 0.2 μm.
It has been clarified that the polycrystal silicon film has its limitation in order to increase the operation speed or improve performance in a display device such as a liquid crystal display device or the organic electroluminescent display device. This is because a large number of crystal grain boundaries existing in an active layer remarkably increase fluctuations in a threshold voltage (Vth) of the TFT, and remarkably degrade operation characteristics in a case where the TFT is prepared using polycrystal silicon. Therefore, it has been demanded that the crystal grain boundaries of the active layer of each TFT be controlled or that crystals be grown to have large particle diameters to thereby remove the crystal grain boundaries.
The present inventors have investigated a method of setting a crystal diameter to be sufficiently larger than a TFT size and controlling generated positions of the crystal grains to thereby remove the crystal grain boundary from the active layer of the TFT. In this method, an optical device (hereinafter referred to as the phase shifter) for modulating a phase of the laser light is inserted midway in an optical path in which the amorphous silicon film is irradiated with the laser light, and a light intensity distribution of the laser light on the amorphous silicon film is adjusted into an appropriate shape to thereby increase the grain diameters (lateral crystal growth). As a result, a technology has been developed which is capable of controlling the position of a silicon single crystal having a large grain diameter of about two to seven microns to thereby realize lateral crystal growth. Furthermore, it has been found that in order to stably crystallize the film having a desired grain diameter, the light intensity distribution of the laser light with which the amorphous silicon film is irradiated in a micro region having a submicron level is particularly important for the crystallization in which the positions of the crystal grains having large diameters are controlled.
Since a light intensity gradient on the surface of the substrate to be treated is an important factor for the growth of the crystal grains, a sectional structure and a surface state of the phase shifter for forming the light intensity distribution are important. Additionally, particles (dust) easily stick to an uneven surface of the phase shifter. This is because the phase shifter is generally made of an insulating material such as glass, static electricity is therefore frequently generated, and, as a result, the particles in the atmosphere are attracted.
Since the stuck particles interrupt the laser light or disturb phase information, the following problem arises. That is, since the light intensity distribution is disturbed by the particles, unintended drop or rise of the light intensity distribution is generated. As a result, the lateral growth of the crystal whose position has been controlled is obstructed. When the particles are burnt on the surface of the phase shifter by irradiation with high-energy laser, the life of the phase shifter is reduced. It is considered that a device be devised in order to remove the particles (e.g., air is sprayed to the surface of a mask), but it is not possible to easily remove the particles stuck onto an uneven portion by the static electricity.